Exhaust-gas probes, which are arranged rearward of a catalytic converter, serve essentially two purposes: they are applied to supplement the lambda control which is essentially based on the signal of an exhaust-gas probe arranged forward of the catalytic converter. In addition, the exhaust-gas probes make possible a monitoring of the conversion capability of the catalytic converter. Both functions are described in German patent publication 3,500,594 which corresponds to U.S. patent application Ser. No. 06/877,882, filed Jan. 10, 1986, abandoned.
It has been shown that the results of monitoring the catalytic converter is influenced by the state of deterioration of the exhaust-gas probe rearward of the catalytic converter.